The offshore production of oil and gas requires the use of substantially vertical, fluid-carrying pipes known as "flowlines" to convey fluids from a subsea wellhead to the water surface. Frequently, a number of flowlines are contained within a vertical tension member called a "riser." The upper end of the riser is connected to a floating vessel or buoy which is subject to lateral movement due to wind, waves, and ocean currents. To compensate for this lateral movement of the vessel or buoy about the stationary subsea wellhead, articulated joints are typically installed between the riser and the subsea wellhead. Swivels or articulated joints are also installed in the flowlines to allow the riser to move without damaging the flowlines. For example, one application of swivels in flowlines is disclosed in U.S. Pat. No. 4,318,423 to DeGraaf (1982).
Swivels and articulated joints in risers and other fluid-carrying systems must be adequately sealed so as to prevent leakage of the fluids into the ambient surroundings. Such fluids will be hereinafter referred to as "production fluids". Typically, pliable elastomeric or plastic fluid seals are utilized between the moving parts of a swivel or articulated joint to prevent leakage of the production fluid. Such pliable fluid seals are used because they form a more effective, leakproof seal than do harder metallic or composition seals. Successful operation of the fluid-carrying system requires that the fluid seals be reliable over the design life of the fluid-carrying system.
The demands on existing fluid seal technology have increased as the quest for crude oil and gas extends into newly discovered reservoirs. Such reservoirs are often located at depths far below the earth's surface, and fluids from such reservoirs are often produced at high temperatures and pressures. For example, deep gas wells may produce fluids at temperatures higher than 450.degree. F. and pressures exceeding 10,000 pounds per square inch (psi). In addition, the fluids produced from such reservoirs are frequently "sour" fluids which contain high concentrations of hydrogen sulfide, carbon dioxide, and other contaminants.
Although fluid seals have been developed which have good resistance to deterioration induced by the chemical action of a sour fluid, these "product seals" tend to deteriorate when subjected to high temperatures and excessive pressures. For example, certain product seal elastomers resistant to chemical deterioration soften as the temperature of the product seal is increased. This softening reduces the tensile strength of the product seal and reduces its ability to resist damage due to excessive pressure. Conversely, other product seal elastomers become brittle at high temperatures. This embrittlement tends to cause the product seal to crack. Furthermore, such embrittlement reduces the resiliency of the product seal which lessens its sealing effectiveness.
Other product seal elastomers such as Fluorocarbons and Perfluoroelastomers which are resistant to chemical deterioration are also resistant to high temperatures. However, such product seals tend to be susceptible to damage due to excessive fluid pressures. The pressure tends to extrude the product seals into the sealing gaps between the moving parts of the swivel or articulated joint and may rupture the product seals. Pressure extrusion of such product seals also increases the wear of the product seals where they contact the sealing lip of the swivel or articulated joint.
Certain plastics are able to withstand the combination of high temperatures, high pressures, and corrosive environments. However, the sealing efficiency of fluid seals made from such plastics is limited. This is because fluid seals made from such plastics tend to be harder than elastomers and do not readily conform to slight irregularities of the mating surface of a swivel or articulated joint. As a result, plastic fluid seals do not seal as effectively as elastomers and often leak.
Fluid seals which can adequately withstand the combination of a corrosive fluid produced at high temperatures and excessive pressures while providing an effective, leakproof seal have not been developed. Thus, a need clearly exists for a sealing system capable of providing a reliable seal when exposed to the combination of these conditions.
Various sealing techniques have been developed to extend the useful life of fluid seals. However, each of these techniques has certain limitations. One such technique utilizes a second, redundant seal behind the first seal to seal the gap between the moving elements of the swivel or articulated joint. If the first seal fails, the second seal will prevent leakage until it also fails. While the second seal reduces the possibility of low pressure leakage, a failure of the first seal resulting from a sudden pressure increase will often rupture the second seal as well. Even if the second seal does not immediately fail, it will thereafter be exposed to the entire pressure of the production fluid and the sealing redundancy desired will no longer exist.
A second technique to extend the useful life of fluid seals uses back-up rings composed of metal or other hard compounds to prevent the softer fluid seal from being extruded into the sealing gap. The back-up ring is placed behind the fluid seal to reduce the size of the sealing gap. This technique combines the advantages of a soft fluid seal, which forms a more effective seal at low fluid pressures than does a more rigid fluid seal, with those of a hard back-up ring, which has a superior resistance to pressure extrusion. However, the use of a back-up ring is limited when the back-up ring is exposed to high temperatures. At high temperatures, the back-up ring will expand. Because the sealing gap between the back-up ring and the moving element of the swivel or articulated joint must be very small in order to prevent extrusion of the fluid seal, thermal expansion of the back-up ring may cause it to seize against the moving element. This will increase the wear and abrasion of the back-up ring and the moving element and will ultimately cause failure of the swivel or articulated joint. Reducing the diameter of the back-up ring to avoid this seizure will increase the size of the sealing gap that the back-up ring is designed to reduce.
A third technique to extend the life of fluid seals uses the combination of a floating piston seal, a stationary back-up seal, and an intermediate hydraulic fluid disposed between the seals. The piston seal is exposed to the production fluid on one side and the hydraulic fluid on the other. The production fluid pressure displaces the piston seal which in turn compresses the hydraulic fluid. The intermediate fluid is prevented from leaking into the ambiance by the back-up seal. See, e.g., E. G. & G. Sealol Thrust ring C-56839. However, a floating piston seal is limited in certain applications. While a floating piston seal eliminates the differential pressure acting on the piston seal, the system relies on movement of the piston seal and is not adaptable to an application where a stationary seal is required. In addition, the sealing effectiveness of the piston seal will be limited by variations in the temperature of the production fluid. At low temperatures, the piston seal will contract and the production fluid will leak into the "clean" hydraulic fluid. At higher temperatures, the piston will expand and seize against the walls of the cylinder.
Aside from the use of redundant seals, back-up rings, and floating piston seals to extend the life of the sealing system, certain other techniques have been developed to pressure balance the moving elements of an articulated joint. For example, in a ball and socket joint, the pressure of the production fluid will tend to force the wear surfaces of the ball and socket elements together. This thrusting force can damage the ball and socket elements by increasing the friction which abrades the wear surfaces. To reduce this thrusting force, a well-known application of the law of hydrostatics utilizes a hydraulic press, or pressure multiplier, to produce a force on the ball and socket elements which opposes the production fluid thrusting force. See, for example, U.S. Pat. Nos. 3,479,061 to Smookler et al (1966) and 3,746,372 to Hynes et al (1973) which disclose the use of a pressure multiplier to pressure balance ball and socket elements. In each of these patents, pressure from the production fluid is directed by a pressure multiplier to pressurize a hydraulic fluid. The hydraulic fluid, which is isolated from the production fluid by the pressure multiplier, is directed against the thrusting element of the joint to counterbalance the production fluid thrusting force. Back-up fluid seals located in the sealing gap between the ball and socket elements prevent the pressurized hydraulic fluid from leaking into the ambient surroundings. While the pressurized hydraulic fluid balances the production fluid thrusting force, the pressure acting on the back-up fluid seals is magnified rather than reduced. Thus, the objective of reducing the pressure which acts on the fluid seals is not accomplished.
While sealing systems have been developed to extend the life of fluid seals which are exposed to sour production fluids being transported at excessive temperatures and pressures, the effectiveness of each sealing system is subject to certain limitations. A need therefore exists for an improved seal pressure reduction system that can extend the life of a sealing system.